Comment form validation errors persist through routing

[Sija]

Steps to reproduce (while logged in):

• Go to some article page
• Click the Post Comment button (without providing any comment text)
• You'll see the Comment on this post: can't be blank message below the comment field
• Navigate away
• Go to an article page again (can be the same or other)
• You'll still see the Comment on this post: can't be blank message